Joy-
I admit to not having read the most recent AMHA rulebook, nor the AMHR one, so I'd recommend checking whichever or both you want to compete under for any guidance, but at least w/ AMHA, I don't recall there being any 'specified' distances. That said...when I was designing courses,I would walk and trot my obstacle horse where I could then measure the stride at each gait, and set the cavalleti/ground poles at the distance of the stride...or at a good average of several horses' strides, as measured.
As for the L...generally, the closer together, the tougher to complete successfully! Realistically, seems that @ 22-24" inside distance was fairly 'doable', taking it down to @ 18' inside to inside made it a tougher, more 'advanced' course.
Just my experience...
Margo